1. Field of the Invention
The present invention relates to a noise removal apparatus, and more particularly to a noise removal apparatus mounted in a vehicle or the like.
2. Description of the Background Art
A radio communication apparatus includes a lot of electrical circuits. Further, the radio communication apparatus includes a highly sensitive reception section so as to receive a communication signal of a high level. Therefore, as is conventionally known, a signal received by the radio communication apparatus includes not only the communication signal but also a noise (hereinafter, referred to as an internal noise) generated in the electrical circuits of the radio communication apparatus itself. Therefore, various methods have been conventionally devised for removing the internal noise from the signal received by the radio communication apparatus.
FIG. 21 is a block diagram illustrating an outline of a configuration of a radio communication apparatus disclosed in Japanese Laid-Open Patent Publication No. 2004-304670. The radio communication apparatus 210 shown in FIG. 21 includes: a first antenna 211; a second antenna 212; a first front end 213; a second front end 214; a first baseband processor 215; a second baseband processor 216; a calculation section 217; and an amplification wave-detector 218.
The first antenna 211 receives a signal including a desired communication signal and an internal noise. The second antenna 212 receives an internal noise. The first front end 213 frequency-converts, to a baseband signal, the signal received by the first antenna 211, and outputs, to the first baseband processor 215, the baseband signal obtained by the frequency-conversion. The second front end 214 frequency-converts, to a baseband noise signal, the internal noise received by the second antenna 212, and outputs, to the second baseband processor 216, the baseband noise signal obtained by the frequency-conversion. The first baseband processor 215 normalizes an amplitude of the baseband signal outputted by the first front end 213, and converts, to a digital baseband signal, the baseband signal having its amplitude normalized. The second baseband processor 216 normalizes an amplitude of the baseband noise signal outputted by the second front end 214, and converts, to a digital baseband noise signal, the baseband noise signal having its amplitude normalized.
The calculation section 217 performs, as a preprocessing, a process of storing noise difference information. The calculation section 217 removes, when performing communication, the internal noise from the digital baseband signal by using a noise differential signal having been previously stored, so as to output, as an estimated received signal, the digital baseband signal which does not include the internal noise. The preprocessing and an operation for communication which are performed by the calculation section 217 will be described below in detail.
The amplification wave-detector 218 controls an amplification rate used by each of the first baseband processor 215 and the second baseband processor 216, in accordance with a signal intensity of the estimated received signal outputted by the calculation section 217 so as to normalize, in an appropriate manner, an amplitude of the digital baseband signal outputted by the first baseband processor 215, and an amplitude of the digital baseband noise signal outputted by the second baseband processor 216.
Next, the preprocessing performed by the calculation section 217 will be described. The calculation section 217 performs the preprocessing in a state where the radio communication apparatus 210 receives neither a desired communication signal nor a noise (hereinafter, referred to as an external noise) generated outside the radio communication apparatus 210. Thus, the signal received by the first antenna 211 includes only the internal noise. Therefore, the digital baseband signal outputted by the first baseband processor 215 includes only the internal noise. The calculation section 217 stores, as the noise difference information, a difference between the digital baseband signal and the digital baseband noise signal. That is, the preprocessing is a process of previously storing information representing a difference between a level of the internal noise received by the first antenna 211 and a level of the internal noise received by the second antenna 212.
Next, an operation for communication performed by the calculation section 217 will be described. The calculation section 217 calculates, when performing communication, a difference between the noise difference information having been previously stored and the baseband noise signal outputted by the second baseband processor 216 (hereinafter, a calculation result will be referred to as an estimated internal noise). Thus, the calculation section 217 is allowed to estimate a level of the internal noise included in the baseband signal outputted by the first baseband processor 215. Next, the calculation section 217 subtracts the estimated internal noise from the baseband signal outputted by the first baseband processor 215, and outputs, as the estimated received signal, the signal obtained by the subtraction. Thus, the calculation section 217 removes only the internal noise from the signal received by the first antenna 211, thereby allowing only the desired communication signal to be outputted.
However, the conventional radio communication apparatus needs to have the second antenna 212 for receiving only the internal noise, in addition to an antenna for performing communication, so as to remove the internal noise. Therefore, a configuration of the conventional radio communication apparatus is complicated. Further, a user of the conventional radio communication apparatus needs to set the second antenna 212 at a position sufficiently near a source from which the internal noise is generated such that the conventional radio communication apparatus allows the calculation section 217 to accurately estimate the internal noise included in the signal received by the first antenna 211. However, when the user is not able to locate the source from which the internal noise is generated, the user is not allowed to set the second antenna 212 at a position sufficiently near the source from which the internal noise is generated. That is, when the user is not able to locate the source from which the internal noise is generated, the conventional radio communication apparatus is not able to accurately estimate the internal noise included in the signal received by the first antenna 211.
Further, the aforementioned conventional radio communication apparatus has one second antenna 212 for receiving the internal noise. However, when there are a plurality of sources from which noises, such as the internal noise and the external noise, are generated, the user needs to additionally set, depending on the number of the sources from which the noises are generated, antennas for receiving the noises, so as to remove the noise in a satisfactory manner. Further, it is necessary to additionally provide the front ends and the baseband processors depending on the number of the antennas to be additionally set.
In general, the radio communication apparatus is fabricated by incorporating therein a lot of electrical circuits of various types, and therefore a plurality of sources from which noises are generated are included in the radio communication apparatus, and it is difficult to accurately locate the sources from which the noises are generated. Accordingly, it is impractical to remove a noise by using the conventional radio communication apparatus.